A D/A converter using a delta-sigma modulation circuit has been known as means for converting a digital signal into an analog signal. An example of a conventional D/A converter is shown in FIG. 5. In FIG. 5, a digital input 401 is PCM data of a long word length, for example, 16 bits. Noise shaping is applied to the digital input 401 by a delta-sigma modulation circuit 402, and the digital input 401 is converted into digital data of several bits. The output of the delta-sigma modulation circuit 402 is put into a local D/A conversion circuit 407 to be an analog output 408.
Regarding the operation of the delta-sigma modulation circuit 402, the digital input 401 is fed into an adder 403 and is summed up with a negative feedback signal from a delay unit 406, and then is integrated by an integrator 404. The output of the integrator 404 is put into a quantizer 405, and is re-quantized into about several bits, and then is put into the local D/A conversion circuit 407. The output of the quantizer 405 is fed into the delay unit 406 as a feedback signal.
The quantizer 405 quantizes the input digital value again according to a quantization reference value. The quantization reference value is set according to the quantization value as a discrete value that can be produced from the quantizer 405. FIG. 6 shows the input and output characteristic of the quantizer 405 of a conventional D/A converter 100b. In the diagram, the black circle marks (●) indicate the quantization reference values. In the diagram, the quantizer 405 re-quantizes the input in five steps from +2 to −2. The quantization reference values corresponding to these five values have linear characteristics (see broken line X). For example, when the input to the quantizer 405 is M, the output of the quantizer 405 is 1, resulting in the quantization error of 0. Similarly, when the input to the quantizer 405 is 2M, the output of the quantizer 405 is 2, resulting in quantization error of 0. The threshold level between adjacent quantization values is an intermediate value to the quantization reference values, and, for example, the threshold value between quantization value 0 and quantization value 1 is M/2.
Generally, in the D/A converter, desirably, there should be no distortion in the signal propagation route, but actually distortion occurs in the local D/A conversion circuit or the analog circuit connected to the rear stage of the local D/A conversion circuit. The following explanation relates to distortion occurring when a pulse width modulation circuit is used as local D/A conversion circuit 407.
The pulse width modulation circuit 407 converts a digital signal of several bits into two values of H level and L level (one bit). The binary signal is ideally a signal as shown in FIG. 7A, but actually has a waveform distortion as shown in FIG. 7B according to analog factors, such as effects of wiring impedance. Occurrence of such waveform distortion deteriorates the characteristics of analog outputs to each output value of the quantizer 405. That is, an ideal analog output has a linear characteristic to the quantizer output (input to the pulse width modulation circuit 407), but the actual analog output has a nonlinear characteristic.
To solve this problem, for example, a signal amplifying device as disclosed in patent document 1 is proposed. According to patent document 1, the distortion quantity is calculated from differences in rising and falling between a pulse width modulated signal and a signal amplified from the pulse width modulated signal to a specified size, and the distortion quantity is subtracted from the quantizer output, so that the distortion can be decreased.
Patent document 1: JP-A-2003-110376